In order to mitigate the noise generated by a jet engine, the jet engine inlet and exhaust duct walls are constructed with noise absorbing material. This is particularly desirable for commercial passenger aircraft, since such aircraft are required to meet stringent government noise regulations. Such noise suppression for commercial passenger aircraft is also desirable so as to enhance the comfort of the passengers thereof.
Additionally, many localities have noise regulations which limit the amount of noise that an aircraft may make in order to prevent annoying nearby residents. This sometimes limits the types of aircraft that may utilize a particular airport. It also frequently imposes procedural restrictions upon aircraft which would otherwise be undesirably. For example, aircraft may be required to fly over designated neighborhoods at a prescribed minimum altitude. Flying at such a minimum altitude may undesirably increase fuel consumption and may also possibly accelerate engine wear (particularly when the minimum altitude must be reached immediately after takeoff).
According to contemporary methodology, a noise suppressing material sold under the name DYNAROHR (a registered trademark of Rohr Industries, Inc.), is utilized extensively for this purpose. The DYNAROHR product is formed of a honeycomb material which is disclosed in U.S. Pat. No. 4,379,191, the contents of which are hereby incorporated by reference. This honeycomb material comprises a core having a plurality of open cells. The core is sandwiched between an outer non-porous layer and an inner porous layer. The inner porous layer is in fluid communication with the open cells of the honeycomb material. A microporous sheet material, such as one comprised of finely woven stainless steel cloth, is bonded over the porous sheet and forms a part of the inner surface of the jet engine's inlet duct.
Although such sound suppressing materials and techniques have proven generally useful for their intended purposes, to date no such material has provided the degree of noise suppression desired for future more stringent noise reduction requirements for use in jet engine applications. As such, it is beneficial to provide a structure which substantially mitigates the noise generated by a jet engine, so as to facilitate compliance with governmental regulations and thus enhance passenger comfort and also mitigate the annoyance of people residing near an airport.